The degree of morbidity and mortality associated with atherosclerosis in developed countries is higher than that associated with any other particular disorder, even cancer. The disorder manifests itself in the form of cholesterol deposition in arterial cell walls. The deposition is slow and irreversible and starts at an early age. Clinical symptoms may take years to manifest themselves and are extremely serious; they include coronary heart disease and stroke. Generally, the disease process will have begun long before these clinical manifestations appear.
Because environmental as well as hereditary factors influence the course of the cholesterol deposition and offer means for at least a mitigation of the process, it is desirable to have available a diagnostic technique which provides an early warning of the onset of the deposition. The present technique depends on measuring cholesterol or triglyceride levels in serum, and while these levels are, indeed, capable of accurate measurement, they do not offer the desirable high correlation to true susceptibility. More reliable predictive methods, which rely on detection of atheromatous lesions, use highly invasive procedures, which are sufficiently painful and expensive that they cannot be employed on a screening basis, or even applied to specific groups selected on the basis of family histories. These techniques also offer too little, too late; by the time the atheromatous lesions have appeared, the most effective time for treatment has been passed.
The importance of early detection is made more poignantly evident by the fact that an effective, but inconvenient and unattractive long term treatment is available--i.e., lowering serum cholesterol through consistently controlled diet or use of cholesterol-lowering drugs. Resistance to this approach will be encountered unless it is clear that the "deprivation" is warranted. The problem is not what the treatment should be, but to whom the treatment should be applied.
A technique that inherently offers the advantages of early detection, if its results can be effectively correlated with the disorder to be assessed, is genetic analysis. Since the genomic characteristics of an individual are basically determined, it is assumed, at conception, genetic aberrations which are indicia of later metabolic disorders are an ideal early diagnosis tool. Genetic testing can be routinely conducted using present methodology, as early as the seventh week of fetal life. Over the last ten years, the availability of restriction enzymes and DNA probing techniques has made possible the use of "restriction fragment length polymorphisms" (RFLPs) in such diagnosis. Using the, by now, well established Southern blot hybridization technique (Southern, E., J Mol Biol (1975) 98:503-517), it has been possible successfully to diagnose sickle cell anemia (Kan, Y.W., et al, Proc Natl Acad Sci (USA) (1978) 75:5631); .beta.-thalassemia (Antonarakis, S.E., et al, Proc Natl Acad Sci (USA) (1983) 79:137); type II diabetes (Rotwein, P., et al, Science (1981) 213:1117); familial growth hormone deficiency (Phillips, J.A., III, Banbury Report 14, Cold Spring Harbor Laboratory (1983) pp 305-315); phenylketonuria (Woo, S.L.C., et al, Nature (1983) 306:151); Huntington's disease (Gusella, J.F., et al, Nature (1983) 306:234); and hemophilia B (Gianelli, et al, Lancet (1984) i:239, Grunenbaum, et al, J Clin Invest (1984) 73:1491).
All of the foregoing successes are grounded in identification of a particular polymorphism or polymorphisms which correlates with the disease or disorder in question. It has been calculated that the number of polymorphisms expected in the human genome should be of the order of 10.sup.7, based on an assumed probability of 0.05 for a given nucleotide to be polymorphic; this number being inferred from studies of the human growth hormone, .alpha.I-antitrypsin and .beta.-like globin gene cluster loci (Jeffreys, A. J., Cell (1979) 18:1-10; Oster, H., et al, Am J Hum Gen (1984) 36(suppl) 150S). The challenge is to determine which of these over ten million polymorphisms is associated with a particular disorder, and to devise a procedure to detect it. The present invention has met this challenge with regard to atherosclerosis, and has also identified a pattern of polymorphisms associated with the same location in the genome useful in characterizing particular individuals and patterns of inheritance.